1. Field of the Invention
This invention relates to detection of explosives and more particularly to an ion mobility spectrometry instrument that detects chemicals present as vapors in air or other gases, or liberated as vapors from condensed phases such as particles or solutions.
2. Description of Related Art
IMS instruments operate on the basis of the time taken by ionized molecules to move through a gas-filled drift region to a current collector while under the influence of an electric field. The ions are created in a gas-filled region called the ion source, which is connected to the drift region through an orifice or a barrier grid. The ion source may use any of a variety of techniques to ionize atoms and molecules. One or more flowing streams of gas enter the ion source through one or more orifices, and the gas may exit through one or more different orifices. At least one of the flowing gas streams entering the ion source includes gas that has been sampled (the xe2x80x9csample gasxe2x80x9d) from the surrounding atmosphere or other source of vapor to be analyzed.
In same cases, the process of taking a sample begins with an operator rubbing an absorbent substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This intermediate absorber is then brought to the vicinity of the sampling orifice of the IMS. The method of concentrating using an absorbent substance is deficient in that it tends to be relatively slow to implement and is subject to variations in the skill of the operator. Additionally, while the absorber is relatively low in cost, the process of taking a great many samples becomes expensive in that the absorber generally should only be used once to ensure consistent results.
The quantity of particles of the target substance on the target surface is usually very small, often corresponding to only nanograms or even picograms of particles per square centimeter. The IMS must be very sensitive to identify a positive signal from evaporated target molecules when the initial concentration and surface area of target particles is so small.
A sampling method that is employed is to provide a gas pump, which draws the sample gas into the ion source through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of the ion source. The partial vacuum is transmitted through the confines of the ion source and appears at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the IMS. The operator places a sample in the near vicinity of this external sampling orifice, and the ambient vapor is drawn into the gas flow moving towards the ion source.
The ion source of the IMS provides a signal that is approximately proportional to the concentration of target molecule vapor. This concentration is further dependent on the equilibrium vapor pressure of the target molecule, the temperature of the target molecule where it is emitting the vapor, the total flow rate of non-target gas that dilutes the target vapor, and possible adsorption losses on surfaces of the gas sampling system. Existing systems that utilize absorbent surface concentration sometimes employ an oven to greatly warm the absorbent material, often up to 200xc2x0, and thereby increase the target vapor concentration.
In some circumstances, it is desirable for IMS instruments to be able to sample vapors at a distance from the external sampling orifice. Examples may include, but not be limited to, sampling of vapor from complex surfaces that contain many holes, crevices, or deep depressions, textured materials such as cloth, people and animals that prefer not to be rubbed by absorbent material, large three dimensional objects, surfaces that must be sampled in a short time, and surfaces in which surface rubbing by human operators is inconvenient or expensive. In addition, it has been observed that the sampling orifice may become contaminated with vapor-emitting particles if the sample inadvertently contacts the orifice. Such contamination is particularly difficult to remove in a short period of time, thus preventing continuous operation of the instrument. Such contamination could be avoided if vapors are sampled at a distance from the sampling orifice.
The distance where vapors may be sampled beyond the sampling orifice may be increased by increasing the sample gas flow rate, i.e., increasing the pumping speed. However, besides the interference with the performance of the ion source of the IMS caused by high velocity flow, this method dilutes the concentration of the desired sample vapor by mixing in a much larger volume of ambient gas. Therefore, the sensitivity of the IMS may decline if the sample gas flow rate is increased excessively.
Warming surfaces at a distance using an oven is generally not very efficient. While warmed gas can be blown onto a distant surface, for example with a xe2x80x9cheat gunxe2x80x9d, when the target surface is a living person or animal, this may not be an acceptable option. Additionally, many surfaces cannot tolerate excessive heating and may be damaged.
According to the present invention, an explosive detection system includes a sampling orifice that receives sampled gas, a fluid rotator that creates a cyclonic gas flow beyond the sampling orifice, an ion source, coupled to the sampling orifice, that generates ions corresponding to the sampled gas, a drift tube having the ion source coupled to a first end thereof, and a detector coupled to an other end of the drift tube, where the detector detects in the sampled gas the presence of ions associated with explosives. The cyclonic gas flow may have an outer rotary flow about an axis substantially parallel to the central axis of the sampling orifice and an inner flow substantially parallel to the central axis of the sampling orifice. The drift tube may operate at substantially ambient gas pressure. A gas pump may draw a gas flow through the sampling orifice and generate a vacuum within 50 millimeters of mercury (50 Torr) of the substantially ambient gas pressure. The fluid rotator may include at least one vane. The fluid rotator may include a rotation-inducing orifice surrounding the sampling orifice. The inside surface of the rotation-inducing orifice may deflect a gas flow into a cyclonic gas flow. The explosive detection system may further include a gas pump connected to the rotation-inducing orifice that creates a cyclonic gas flow. The explosive detection system may include a precipitator that removes at least a portion of any entrained particles within the gas flow into the sampling orifice. The precipitator may be an electrostatic precipitator. The electrostatic precipitator may include a cathode disposed on or near the drift tube, the cathode applying a voltage greater than 3000 Volts. The axis of the cyclonic gas flow may rotate about a rotation axis perpendicular to its central axis. The axis of the cyclonic gas flow may rotate about a plurality of rotation axes perpendicular to its central axis.
According further to the present invention, an explosive detection system includes a sampling inlet that receives sampled gas, a heat source, mounted proximal to the gas sampling inlet, the heat source providing photonic emissions to one side of a target proximal to the sampling inlet, an ion source, coupled to the sampling orifice, that generates ions corresponding to the sampled gas, a drift tube having the ion source coupled to a first end thereof, and a detector coupled to an other end of the drift tube, where the detector detects in the sampled gas the presence of ions associated with explosives. The photonic emissions may be substantially in the infrared portion of the spectrum. The source of photon emission may be made to be substantially in the infrared using at least one of a filter, coating, and covering. The source of photon emission may have enhanced emission substantially in the infrared by means of conversion of visible light photons to infrared photons. The photonic emissions may be substantially in the combined visible and infrared portion of the spectrum. The photonic emissions may be substantially in the visible portion of the spectrum. The source of photon emission may be made to be substantially in the visible using at least one of a filter, coating, and covering. The photonic emissions may be provided by at least one of a thermally heated surface, a laser, a light emitting diode, and an electrical discharge in a gas. The source of photon emission may be at least one of: pulsed, keyed in a long pulse, and continuous. The source of photon emission may be separated from the target surface by at least one of a window and a semi-transparent grid.
According further to the present invention, a target sample heating system for an ion mobility spectrometer includes a source of photon emission substantially in the infrared portion of the spectrum, means for concentrating the photon emission into a beam, and means for guiding the photon emission towards a target surface. The source of photon emission may be at least one of: a thermally heated surface, laser, light emitting diode, and an electrical discharge in a gas. The source of photon emission may be at least one of: pulsed, keyed in a long pulse, and continuous. The means for concentrating the photon emission may be at least one of a mirror, lens, and fiber optic wave guide. The means for guiding the photon emission towards a target surface may be at least one of a mirror, lens, and fiber optic wave guide. The means for guiding the photon emission may be moved or tilted while guiding the photon emission. The source of photon emission may be made to be substantially in the infrared using at least one of a filter, coating, and covering. The source of photon emission may have enhanced emission substantially in the infrared by means of conversion of visible light photons to infrared photons. The source of photon emission may be separated from the target surface by at least one of a window and a semi-transparent grid.
According further to the present invention, a target sample heating system for an ion mobility spectrometer includes a source of photon emission substantially in the combined visible and infrared portion of the spectrum, means for concentrating the photon emission into a beam, and means for guiding the photon emission towards a target surface. The source of photon emission may be at least one of a thermally heated surface, a laser, light emitting diode, and an electrical discharge in a gas. The source of photon emission may be at least one of: pulsed, keyed in a long pulse, and continuous. The means for concentrating the photon emission may be at least one of a mirror, lens, and fiber optic wave guide. The means for guiding the photon emission towards a target surface may be at least one of a mirror, lens, and fiber optic wave guide. The means for guiding the photon emission may be moved or tilted while guiding the photon emission. The source of photon emission may be separated from the target surface by at least one of a window and a semi-transparent grid.
According further to the present invention, a target sample heating system for an ion mobility spectrometer includes a source of photon emission substantially in the visible portion of the spectrum, means for concentrating the photon emission into a beam, and means for guiding the photon emission towards a target surface. The source of photon emission may be at least one of a thermally heated surface, a laser, light emitting diode, and an electrical discharge in a gas. The source of photon emission may be at least one of: pulsed, keyed in a long pulse, and continuous. The means for concentrating the photon emission may be at least one of mirror, lens, and fiber optic wave guide. The means for guiding the photon emission towards a target surface may be at least one of a mirror, lens, and fiber optic wave guide. The means for guiding the photon emission may be moved or tilted while guiding the photon emission. The source of photon emission may be made to be substantially in the visible using at least one of a filter, coating, and covering. The source of photon emission may be separated from the target surface by at least one of a window and a semi-transparent grid.
According further to the present invention, a sampling system for an IMS includes a gas sampling inlet that samples vapors from a target and provides the vapors to the IMS and a heat source, mounted proximal to the gas sampling inlet, the heat source providing photonic emissions to the target in connection with the inlet sampling vapors. The photonic emissions may be substantially in the infrared portion of the spectrum. The source of photon emission may be made to be substantially in the infrared using at least one of a filter, coating, and covering. The source of photon emission may have enhanced emission substantially in the infrared by means of conversion of visible light photons to infrared photons. The photonic emissions may be substantially in the combined visible and infrared portion of the spectrum. The photonic emissions may be substantially in the visible portion of the spectrum. The source of photon emission may be made to be substantially in the visible using at least one of a filter, coating, and covering. The photonic emissions may be provided by at least one of a thermally heated surface, a laser, a light emitting diode, and an electrical discharge in a gas. The source of photon emission may be at least one of: pulsed, keyed in a long pulse, and continuous. The source of photon emission may be separated from the target surface by at least one of a window and a semi-transparent grid.
According further to the present invention, a gas sampling system for an ion mobility spectrometer includes a first gas pump providing a gas flow at a partial gas vacuum compared to ambient gas pressure, a second gas pump providing a gas flow at a partial gas pressure compared to the ambient gas pressure, a first orifice for the partial gas vacuum which is external to the ion mobility spectrometer, tubulation means connecting the first orifice to the ion mobility spectrometer, a second orifice for the partial gas pressure which is concentric and external to the first orifice, and gas deflection means for inducing a rotational cyclonic motion of the gas flow from the second orifice. The partial gas vacuum may be within 50 millimeters of mercury (50 Torr) of the ambient gas pressure. The partial gas pressure may be within 50 millimeters of mercury (50 Torr) of the ambient gas pressure. The gas deflection may be provided by vanes or by the inside surface of the second orifice.
According further to the present invention, a gas sampling system for an ion mobility spectrometer includes a first gas pump providing a gas flow at a partial gas vacuum compared to ambient gas pressure, a second gas pump providing a gas flow at a partial gas pressure compared to the ambient gas pressure, a first orifice for the partial gas vacuum which is external to the ion mobility spectrometer, tubulation means connecting the first orifice to the ion mobility spectrometer, a second orifice for the partial gas pressure which is concentric and external to the first orifice, gas deflection means for inducing a rotational cyclonic motion of the gas flow from the second orifice; and electrostatic field means for precipitating particles inside the tubulation means. The partial gas vacuum may be within 50 millimeters of mercury (50 Torr) of the ambient gas pressure. The partial gas pressure may be within 50 millimeters of mercury (50 Torr) of the ambient gas pressure. Gas deflection may be provided by vanes or by the inside surface of the second orifice. The electrostatic means may be provided by a cathode disposed substantially on the axis of the tubulation with an applied voltage greater than 3000 Volts.
According further to the present invention, a gas sampling system includes an ion mobility spectrometer having a sampling orifice and a fluid rotator that creates a cyclonic gas flow beyond the sampling orifice, the cyclonic gas flow having an outer rotary flow about an axis substantially parallel to the central axis of the sampling orifice and an inner flow substantially parallel to the central axis of the sampling orifice. The ion mobility spectrometer may operate at substantially ambient gas pressure. A gas pump may draw a gas flow through the sampling orifice and generate a vacuum within 50 millimeters of mercury (50 Torr) of the substantially ambient gas pressure. The fluid rotator may include at least one vane. The fluid rotator may include a rotation-inducing orifice surrounding the sampling orifice. The inside surface of the rotation-inducing orifice may deflect a gas flow into a cyclonic gas flow. The gas sampling system may also include a gas pump connected to the rotation-inducing orifice that creates a cyclonic gas flow. The gas sampling system may also include a precipitator that removes at least a portion of any entrained particles within the gas flow into the sampling orifice. The precipitator may be an electrostatic precipitator. The electrostatic precipitator may include a cathode disposed on or near the drift tube, the cathode applying a voltage greater than 3000 Volts. The axis of the cyclonic gas flow may rotate about a rotation axis perpendicular to its central axis. The axis of the cyclonic gas flow may rotate about a plurality of rotation axes perpendicular to its central axis.
According further to the present invention, a compound gas sampling system for an ion mobility spectrometer, includes a plurality of gas sampling systems as described herein, the gas sampling systems arranged so that adjacent cyclonic flows rotate in opposing directions.
The invention applies to an ion mobility spectrometer that uses an external sampling orifice to draw in vapors to be analyzed. A method for warming a distant target surface is described using at least one of several techniques. The goal is to heat the target surface in a manner such that the action of heating is unobtrusive, perhaps invisible, the sampled portion of the surface is warmed at least 5xc2x0 C., and only the surface is warmed, not the bulk of the target material. These conditions may be accomplished using one or more infrared light sources, one or more visible light sources, or a mixture of the two. A light source that is substantially in the infrared portion of the spectrum has the advantage that it is largely invisible to the eye, except for a slight reddish appearance. However, brighter light sources, that warm the surface more quickly, can be produced more easily using visible light. Infrared wavelengths are generally considered to be longer than 700 nanometers and shorter than 100 micrometers. Visible wavelengths are generally considered to be in the range of 700 nanometers to 300 nanometers. Most sources of visible light produce some small percentage of ultraviolet light less than 300 nanometers and some small percentage of infrared light. Most light sources, except lasers, produce broad distributions of wavelengths, and a source is considered to be a visible light source if the peak of its distribution is in the visible range of wavelengths.
It is preferable to utilize means for guiding and concentrating the photon beam from the light source towards the place on the target surface where gas sampling is most efficiently being performed in order to minimize the power consumption, heat primarily the target surface of interest, and maximize the lifetime of the light source. The means may be in the form of one or more lenses, one or more mirrors, fiber optic cable, or some combination of these. An example would consist of a parabolic mirror combined with a nearly point source of infrared light. With the point source situated near to the focal point of the mirror, a substantially parallel infrared beam results, which can then be directed at the desired location on the target surface.
The source of light may be continuous or pulsed. Pulsed light has the advantage of conserving energy and avoiding overheating of the target surface. A desirable feature is to turn off the light source when not in use, but a continuous output light source often requires time to come to stable operating conditions. An alternate embodiment would be to combine a shutter with the continuous output light source in order to simulate a pulsed source. Equivalently, the source of light may be pulsed with a long duration on the order of seconds, sometimes referred to as xe2x80x9ckeyedxe2x80x9d.
The interaction of the light radiation with the particles of target material depends on the wavelength of radiation employed. At some wavelengths, the target particles may substantially reflect the incident radiation, thus not absorbing energy and becoming warmed. Heating is then accomplished indirectly by using the incident radiation to warm the surface on which the target particles are attached with heat being transferred to the target particles by conduction, convection, or conversion of the incident wavelength to one that is substantially longer where the target molecules are more absorptive.
There are many well-known sources of infrared and visible light that may be utilized. A hot wire, possible heated electrically, may be used for infrared emission. The wire temperature may be near 800xc2x0 C. to 850xc2x0 C. when operated in air. An example of a pulsed visible light source is a xenon flash lamp, in which the pulse duration in one embodiment is approximately 10xe2x88x924 seconds. Laser light sources are available both pulsed and continuous at single wavelengths covering much of the infrared and visible light spectrum.
The invention applies to an ion mobility spectrometer that uses an external sampling orifice to draw in vapors to be analyzed. In addition to this existing orifice, a coaxial orifice is provided which emits gas towards the object to be sampled. The emitted gas is further deflected such that it is induced to move in a circular flow about the axis of the external sampling orifice. A further component of the motion is a net velocity away from the external sampling orifice. This type of flow is often referred to as a cyclone. The spinning motion results in a radially-outward directed centrifugal force that restrains the emitted gas flow from immediately being drawn radially inward into the partial vacuum of the external sampling orifice. Eventually, friction with the surrounding ambient gas will slow the emitted gas sufficiently that it will be drawn into the partial vacuum at some distance from the external sampling orifice. Depending on the flow of the emitted gas, this distance can be varied from near the external sampling orifice (low flow) to far from the external sampling orifice (high flow). The cyclonic motion in effect creates a tube consisting of a wall of moving gas that behaves like an extension of the tube that formed the external sampling orifice.